This invention relates to a system for protecting electrically powered motors against thermal overload.
In the past thermal protection of electrically powered motors was derived by detecting motor current and when the time product of the motor current exceeded a predetermined threshold, the motor was shut down. An example of one such system for preventing thermal overload is disclosed in Linkus U.S. Pat. No. 3,821,602 wherein a fuse is positioned in circuit with each of the three phase windings of the motor. When the current overload exceeded a predetermined time-current product, as determined by the fuse characteristics, the fuse blew and the motor would be shut down. More recently, sophisticated techniques have been developed for determining the time-current product in AC motors. Thus, in Cameron U.S. Pat. No. 4,052,625 there is disclosed a current detection system which uses an SCR and a current sensing resistor for detecting the current flow into a DC motor. When the current exceeds a predetermined threshold as determined by a comparator, an output is provided to an integrating circuit. If the increased load current exists for a predetermined sufficient duration, a second comparator provides an output which deenergizes a control switch to shut off current to the motor.
It has been found, however, that thermal overload is dependent not only upon current input to the motor, but also, in a variable speed drive system, is dependent upon the level of fan cooling. When, for example, in an induction motor, a loss of fan cooling occurs, such as might happen when the motor is operating at reduced speeds, the level of current required to drive the motor into a thermal overload condition is reduced. Therefore, the prior art thermal overload protection circuits which assumed full speed fan cooling did not adequately protect such variable speed motors at reduced motor speeds.
Sons et.al. U.S. Pat. No. 3,912,976 discloses a circuit wherein the motor current values are determined and adjusted depending upon the speed of the motor. Thus at a first speed, the current itself is integrated with respect to time to determine whether the motor has reached its thermal overload stage. If the motor speed decreases below a predetermined level, a determination is made as to whether thermal overload has been reached at a lower current level. While this system is, in a general sense, sensitive to motor speed, it does not provide a control wherein the thermal overload level can be determined by continuously taking into consideration the variation of motor speed with respect to a predetermined rate of speed. Accordingly, there is a need for an improved motor protection system, particularly for variable speed motors.